1. Field of the Invention
The present invention relates to an exposure apparatus in which with at least a portion between a projection optical system and a substrate being filled with a liquid, a pattern is exposed onto the substrate via the projection optical system and the liquid, and to a device manufacturing method that uses the exposure apparatus.
2. Description of Related Art
Semiconductor devices and liquid crystal display devices are manufactured through the so-called photolithography technique, by which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in the photolithography process has a mask stage that supports the mask and a substrate stage that supports the substrate, and while successively moving the mask stage and the substrate stage, transfers the mask pattern, via a projection optical system, onto the substrate. In recent years, to address the increasingly high integration of device patterns, increasingly high resolution of the projection optical system has been desired. The shorter the exposure wavelength used is, and, also, the larger the numerical aperture of the projection optical system is, the higher the resolution of the projection optical system becomes. For this reason, the exposure wavelength used for the exposure apparatus is becoming shorter and shorter year by year, and the numerical aperture of the projection optical system is also becoming larger and larger. In this context, the presently dominant exposure wavelength is 248 nm from a KrF excimer laser, but a still shorter wavelength of 193 nm from an ArF excimer laser is now gradually being put to practical use. In addition, when performing exposure, the depth of focus (DOF) is an important factor along with the resolution. The resolution R and the depth of focus δ are respectively expressed by the following formulas:R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)where λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. It can be seen from formulas (1) and (2) that if, to enhance the resolution R, the wavelength λ is made shorter and the numerical aperture is made larger, then the depth of focus δ becomes narrower.
When the depth of focus δ becomes too narrow, it becomes difficult to make the substrate surface coincide with the image plane of the projection optical system, and thus there occurs the possibility that the focus margin during the exposure operation will be insufficient. To address this problem, the liquid immersion method, which is disclosed in, e.g., below-described patent documents 1 and 2, has been proposed as a method to make the exposure wavelength shorter in effect and to make the depth of focus broader. This liquid immersion method is designed, by filling the space between the under surface of the projection optical system and the substrate surface with a liquid, e.g., water or organic solvent, to form a liquid immersion region and thus by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times (n is the refractive index of the liquid and is generally about 1.2 to 1.6) of that in the air, improve the resolution and enlarge the depth of focus by approximately n times.
By the way, with respect to the above-mentioned related art, there are problems as described below.
The exposure apparatus disclosed in the PCT International Publication No. WO 99/49504 is configured such that a liquid is supplied and recovered so that a liquid immersion region is formed on a part of a substrate; however, when the liquid is not completely recovered and remains on the substrate, there is the possibility that the remaining liquid vaporizes, thereby the substrate thermally deforms and, after vaporizing, an adhesion trace (so-called water mark) remains on the substrate and adversely affects the pattern to be formed on the substrate. On the other hand, the exposure apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. H10-303114 is configured such that the entirety of a substrate is held in a liquid; however, because the liquid between the projection optical system and the substrate is not replaced so much, there is the possibility that the temperature of the liquid of the liquid immersion region may change and impurities are apt to be contained therein, which may deteriorates the pattern image projected onto the substrate. Thus, there arises the possibility that devices having a desired performance cannot be manufactured.
Furthermore, with respect to the exposure apparatus of the above-mentioned PCT International Publication No. WO 99/49504, when a substrate having experienced an exposure process is conveyed (carried out), with the liquid remaining (adhering) on the surface of the substrate, the remaining liquid drops from the substrate during the conveyance process, which results, for example, the disadvantage that due to the liquid dropped, various devices and members neighboring on the conveyance path rust, or the cleanliness of the ambience in which the exposure apparatus is installed cannot be maintained. Furthermore, the liquid that has dropped (scattered) from the substrate may cause the ambient condition (humidity) change around the exposure apparatus. When the humidity changes, there arises, for example, the problem that the air on the optical path of an optical interferometer used for measuring the stage position fluctuates, the stage measurement cannot be performed accurately, and thus a desired pattern transfer accuracy cannot be obtained. Furthermore, if after the exposure process, a development process, for example, is performed in a state that the liquid still adheres on the substrate, there arises the possibility that devices having a desired performance cannot be manufactured.